chore: merge probe-engine into probe-cli (#201)

This is how I did it:

1. `git clone https://github.com/ooni/probe-engine internal/engine`

2. ```
(cd internal/engine && git describe --tags)
v0.23.0
```

3. `nvim go.mod` (merging `go.mod` with `internal/engine/go.mod`

4. `rm -rf internal/.git internal/engine/go.{mod,sum}`

5. `git add internal/engine`

6. `find . -type f -name \*.go -exec sed -i 's@/ooni/probe-engine@/ooni/probe-cli/v3/internal/engine@g' {} \;`

7. `go build ./...` (passes)

8. `go test -race ./...` (temporary failure on RiseupVPN)

9. `go mod tidy`

10. this commit message

Once this piece of work is done, we can build a new version of `ooniprobe` that
is using `internal/engine` directly. We need to do more work to ensure all the
other functionality in `probe-engine` (e.g. making mobile packages) are still WAI.

Part of https://github.com/ooni/probe/issues/1335

internal/engine/experiment/ndt7/download_test.go

@@ -0,0 +1,145 @@
+package ndt7
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"io"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/gorilla/websocket"
+)
+
+func TestDownloadSetReadDeadlineFailure(t *testing.T) {
+	expected := errors.New("mocked error")
+	mgr := newDownloadManager(
+		&mockableConnMock{
+			ReadDeadlineErr: expected,
+		},
+		defaultCallbackPerformance,
+		defaultCallbackJSON,
+	)
+	err := mgr.run(context.Background())
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+}
+
+func TestDownloadNextReaderFailure(t *testing.T) {
+	expected := errors.New("mocked error")
+	mgr := newDownloadManager(
+		&mockableConnMock{
+			NextReaderErr: expected,
+		},
+		defaultCallbackPerformance,
+		defaultCallbackJSON,
+	)
+	err := mgr.run(context.Background())
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+}
+
+func TestDownloadTextMessageReadAllFailure(t *testing.T) {
+	expected := errors.New("mocked error")
+	mgr := newDownloadManager(
+		&mockableConnMock{
+			NextReaderMsgType: websocket.TextMessage,
+			NextReaderReader: func() io.Reader {
+				return &alwaysFailingReader{
+					Err: expected,
+				}
+			},
+		},
+		defaultCallbackPerformance,
+		defaultCallbackJSON,
+	)
+	err := mgr.run(context.Background())
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+}
+
+type alwaysFailingReader struct {
+	Err error
+}
+
+func (r *alwaysFailingReader) Read(p []byte) (int, error) {
+	return 0, r.Err
+}
+
+func TestDownloadBinaryMessageReadAllFailure(t *testing.T) {
+	expected := errors.New("mocked error")
+	mgr := newDownloadManager(
+		&mockableConnMock{
+			NextReaderMsgType: websocket.BinaryMessage,
+			NextReaderReader: func() io.Reader {
+				return &alwaysFailingReader{
+					Err: expected,
+				}
+			},
+		},
+		defaultCallbackPerformance,
+		defaultCallbackJSON,
+	)
+	err := mgr.run(context.Background())
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+}
+
+func TestDownloadOnJSONCallbackError(t *testing.T) {
+	mgr := newDownloadManager(
+		&mockableConnMock{
+			NextReaderMsgType: websocket.TextMessage,
+			NextReaderReader: func() io.Reader {
+				return &invalidJSONReader{}
+			},
+		},
+		defaultCallbackPerformance,
+		func(data []byte) error {
+			var v interface{}
+			return json.Unmarshal(data, &v)
+		},
+	)
+	err := mgr.run(context.Background())
+	if err == nil || !strings.HasSuffix(err.Error(), "unexpected end of JSON input") {
+		t.Fatal("not the error we expected")
+	}
+}
+
+type invalidJSONReader struct{}
+
+func (r *invalidJSONReader) Read(p []byte) (int, error) {
+	return copy(p, []byte(`{`)), io.EOF
+}
+
+func TestDownloadOnJSONLoop(t *testing.T) {
+	mgr := newDownloadManager(
+		&mockableConnMock{
+			NextReaderMsgType: websocket.TextMessage,
+			NextReaderReader: func() io.Reader {
+				return &goodJSONReader{}
+			},
+		},
+		defaultCallbackPerformance,
+		func(data []byte) error {
+			var v interface{}
+			return json.Unmarshal(data, &v)
+		},
+	)
+	ctx, cancel := context.WithTimeout(context.Background(), 1*time.Second)
+	defer cancel()
+	err := mgr.run(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+type goodJSONReader struct{}
+
+func (r *goodJSONReader) Read(p []byte) (int, error) {
+	return copy(p, []byte(`{}`)), io.EOF
+}